1. Field of the Invention
The invention relates to interferometers and, more particularly, relates to a geometrically-desensitized interferometer (GDI) instrument for surface profiling. Even more particularly, the invention relates to a GDI instrument having an adjustable range of measurement depths and to a method and apparatus for adjusting the range of measurement depths of a GDI instrument. The invention additionally relates to a GDI instrument incorporating measures to enhance the ability of the instrument's imaging device to profile rough surfaces by constraining reflected light to return to the imaging device.
2. Discussion of the Related Art
Optical metrology of surface profiles can generally be divided into two regimes, namely interferometric and geometric. Geometric techniques include triangulation and moire fringe analysis, which involves the projection and imaging of a periodic structure such as a ronchi ruling. Geometric techniques are relatively insensitive to surface roughness and deformations, but are of relatively low resolution--so low, in fact, that they are unsuitable for many applications in which surface profiles must be measured with high precision.
Interferometry, on the other hand, relies on the wave nature of light to ascertain with high precision the surface profile of a test object. A typical traditional interferometer includes a light generator that generates a beam of light, a spatial filter-beam diverger that diverts the light beam into a diverging spherical wavefront, a beamsplitter that diverts part of the diverging spherical wavefront from the filtered beam, and a collimating lens that collimates the wavefront to produce a plano wavefront of coherent light. This wavefront of coherent light is then reflected off test and reference surfaces to produce first and second reflected wavefronts which combine with one another while interfering both constructively and destructively to produce an interference fringe pattern. An imaging device such as a solid state camera receives the recombined wavefronts and acquires images of the interference fringe pattern. The interference fringe pattern then is analyzed to obtain information about the surface profile of the test object.
Fringe pattern analysis for surface profilometery often is performed by the well-known technique of phase shifting interferometry (PSI). In PSI, the height difference between locations on a surface imaged by first and second pixels on the imaging device is determined by first determining a phase difference between light received at the first and second pixels and by then using the phase difference to calculate a height difference. A primary advantage of PSI is that it is highly precise. The vertical height precision for PSI is a fraction (e.g., 1/100) of the optical wavelength of the light source used to conduct the measurement. A second advantage of PSI is that it has good vibration immunity characteristics because phase data is acquired for all pixels simultaneously and because the data acquisition time is relatively short.
Generally speaking, however, conventional PSI approaches can only profile smooth surfaces having relatively small height variations or "surface departures" between adjacent measurement sites. This constraint results from the fact that PSI has a phase ambiguity constraint. Specifically, the maximum physical departure between adjacent measurement sites on the profiled surface must be less than 1/4 of the source wavelength. Stated another way, the maximum phase difference between the reference and test light beams must have an absolute value which is less than .pi.. This constraint, sometimes known as "two .pi. ambiguity", arises because the arctangent function, which is used to convert phase to distance, is only unique within the range of .+-..pi.. Thus, although the use of phase measurements advantageously allows very high precision to be obtained, it disadvantageously limits the maximum surface departure between adjacent measurement sites to one quarter of the source's optical wavelength. A further difficulty with PSI arises when the surface slope is so large that it becomes difficult to resolve or distinguish the interference fringes because the fringe density is too high. Therefore, while PSI interferometetry is much more precise than geometric optical profilometery, it historically has been considered to be ill-suited for use with rough objects or objects having marked surface deformations. Interferometers using PSI analysis therefore historically have not been considered appropriate for some surface profilometery applications.
One interferometric technique that lacks the quarter-wavelength constraint of PSI is the so-called scanning white light interferometry or SWLI. In SWLI, a white light illumination source or, more generally, one which is of a broad-band as opposed to being of a narrow-band generates an interference pattern which contains regions of high contrast for each location on the test surface as a function of scan position. The scan position of high contrast for a given pixel indicates the height of the corresponding location on the test surface. Therefore, by comparing the temporal characteristics of these regions of high contrast with one another, a difference in height between two locations on the profiled surface can be determined. Unlike PSI, SWLI does not calculate height differences based on phase differences, and the PSI phase constraint therefore does not apply to SWLI. The maximum physical departure between adjacent measurement sites on a profiled surface therefore may be much larger with SWLI than with PSI.
However, SWLI has disadvantages of its own that hinders its use in industrial applications. For instance, the field of view is generally no larger than can be accommodated by standard microscope objectives. To function correctly, the imaging device of the instrument must have high resolution when compared to the corresponding interference fringe density. When the field of view of the typical SWLI instrument is increased, the fringe density can easily become difficult to resolve even with very high resolution imaging devices. This problem is especially evident during the profiling of rough surfaces. Moreover, slope tolerance for specular surfaces decreases linearly with the field size, and the speckle effects required for rough-surface measurements are only resolvable if the numerical aperture (NA) of the objective decreases linearly as the field increases. The need to resolve the speckle pattern from rough surfaces is the most discouraging, since the amount of collected light decreases with the square of the NA. The light loss means that larger surfaces require a more powerful illuminator. Worse, the fringe contrast is now a highly variable parameter, and the quality of the measurement depends critically on the balance between the reference and object beam intensities.
Another disadvantage of typical SWLI techniques is that data acquisition is very slow. The slow speed is a consequence of the rapidly varying interference effect as a function of scan position. Accurate measurements require that these variations be recorded in detail, usually at the rate of one measurement per pixel per 75 nm of scan motion. The slow speed creates additional problems such as a high sensitivity to thermal distortions and mechanical strain during measurement.
Still another disadvantage of typical SWLI is its high sensitivity to vibration, which is due in part to the slow data acquisition speed, and in part to the extremely high sensitivity of the interference fringe pattern, which is easily corrupted by very small amounts of vibration. An instrument configured for SWLI analysis generally requires massive mounting fixtures and expensive vibration isolation. Even with these precautions, such instruments are still restricted to relatively vibration-free environments as compared to normal production environments.
Recent years have seen an increased demand for the high speed, high precision metrology of the surface profiles of manufactured parts having large surface departures, i.e., having rough surfaces or surfaces with pronounced surface deformations. A corresponding demand has arisen for the acquisition of data during production as opposed to in the laboratory. For instance, precision products such as hard disks for computer disk drives need to be profiled with high precision, at high speeds, and under conditions in which the test object may be subjected to substantial vibrations during manufacturing processes. Neither traditional PSI techniques nor traditional SWLI techniques are suitable for these purposes. A need therefore has developed for a "desensitized" interferometer that is relatively insensitive to surface roughness and surface deformations, that performs surface metrology with high accuracy and at high speeds, and that is relatively insensitive to vibrations and therefore is well-suited to production-line use.
This need has been met to a large extent by the development of the geometrically-desensitized interferometer (GDI) instrument. A GDI instrument is characterized by the replacement of the beam splitter of the traditional instrument with an optical assembly located between the collimating lens and the test object. The optical assembly, which typically (but not necessarily) comprises a diffraction grating assembly, a hologram, or diffractive optics in combination with conventional optics such as mirrors and lenses, divides the collimated source light into two beams which propagate in two different directions and impinge on the profiled surface at the same location but at different incident angles. The beams reflect from the profiled surface and pass back through the optical assembly in different directions, after which they are recombined. Constructive and destructive interference of the reflected and recombined beams form an interference fringe pattern having an equivalent wavelength A that may be orders of magnitude larger than the source wavelength. As a result, the GDI instrument is much less sensitive to height variations and surface deformations than are traditional interferometers using PSI analysis techniques. Some forms of GDI instruments also are achromatic. That is, the fringe spacing in an interference fringe pattern produced by a GDI instrument is independent of the source wavelength. As a result, and unlike with SWLI interferometers, there is no coherence envelope associated with the source bandwidth. Many disadvantages associated with SWLI such as a limited field of view, a slow acquisition speed, and a high sensitivity to vibration therefore are avoided. The sensitivity of GDI instruments is intermediate conventional interferometry and moire fringe analysis, and is comparable to that obtained with grazing-incidence interferometry. GDI instruments therefore can be used in manufacturing applications and other applications that are unsuitable for traditional interferometry.
A characteristic of GDI instruments is that they exhibit a noticeable decline in fringe contrast with an increase in the effective lateral length of the source light, i.e., in the physical dimension of the source light in a direction which extends laterally along the plane of the optical assembly. As a result, the range of distances from the light generator to the test surface over which a contrast intensity of a minimum acceptable threshold exists varies inversely with the effective lateral length of the source light. Stated another way, the equivalent coherence envelope is inversely proportional to the effective lateral length of the source light. A GDI instrument generating a relatively physically narrow source light can tolerate substantial changes in the distance between the test surface and the optical assembly while retaining acceptable fringe contrast. A GDI instrument generating a relatively wide source light is relatively intolerant of variations in distance between the test surface and the optical assembly.
Since the effective lateral length of the source light varies directly with the effective lateral length of the light generator's discharge aperture, conventional wisdom might be to design a GDI instrument so as to minimize as much as practical the effective lateral length of the discharge aperature, thereby maximizing the width of the equivalent coherence envelope and the effective range of measurement depths. However, it has been discovered that situations exist in which a relatively narrow equivalent coherence envelope and a resultant relatively small range of measurement depths are desirable. These applications include profiling only one surface of an optically transparent element and differentiating between a feature of interest on a test surface such as a groove and an adjacent feature that is separated from the feature of interest by a substantial vertical distance. However, applications in which low coherence operation is desirable are few in comparison to applications in which high-coherence operation is desirable, and designing a GDI instrument specifically for low coherence operation therefore is impractical. Moreover, even if a GDI instrument were designed uniquely for low coherence operation, it would be impossible to switch back and forth between low coherence mode and high coherence mode for focusing or similar operations.
Another problem associated with many optical instruments and even with many GDI instruments is that they may experience difficulty profiling a surface that is so rough that it scatters reflected light to such an extent that the instrument's imaging device cannot generate images with acceptable contrast for interferometric analysis. This problem could be ameliorated if a mechanism were available to in some way constrain reflected light to travel back to the imaging device so as to optimize coherence in the image. No such devices have heretofore been available.